Epoxy graphite composite materials, also known as carbon fiber-reinforced plastics, are very strong, lightweight, high performance materials used in the manufacture of vehicles, sporting goods, and consumer products, among others. The materials can be made in a variety of geometric forms and sizes by layering sheets of carbon fiber cloth into a mold in the shape of the final product followed by filling the mold with epoxy and curing. The type and alignment of the fibers are selected to optimize the strength and stiffness properties of the resulting composite material, and air may be evacuated from the mold prior to curing to enhance structural rigidity. The epoxy, which provides a structural matrix that is strengthened by carbon fibers, is commonly produced from a reaction between a phenol or cresol (e.g., bisphenol-A) and a crosslinking agent (e.g., epichlorohydrin).
Epoxy graphite composite materials are known to have excellent thermal cycling and mechanical fatigue properties. Unfortunately, the photon energy of ultraviolet components of ambient light (e.g., 290-400 nm), including sun light and artificial light sources that illuminate manufacturing facilities, are comparable to the dissociation energies of polymer covalent bonds found in epoxy graphite composite materials.
Ultraviolet (“UV”) light absorbed by epoxy polymers causes photo-oxidative reactions that cause material degradation. UV-mediated polymer chain scission lowers the molecular weight of polymers giving rise to reduced strength and heat resistance. Likewise, UV-crosslinking causes excessive brittleness and results in microstructural defects. Exposure to ultraviolet light can also produce chromophores, which discolor the polymer, and autocatalytic degradation may be established if UV-absorbing chromophores are produced. Various photostabilizers can be added to polymers to inhibit degradation by UV radiation, but exposure to UV radiation may nevertheless result in significant degradation of mechanical properties, especially at high temperatures.
For relatively short periods of UV exposure, usually only changes in surface morphology are observed. However, for extended exposure to UV radiation, an epoxy graphite material may be severely degraded. The black surfaces of an epoxy graphite material exposed to UV radiation exhibit a distinct color change from black to dark green during early stages of UV-degradation. Often, the formation of green color serves as a convenient means for monitoring the degradation process. Changes in surface smoothness may also be visible by the naked eye upon UV irradiation.
The mechanical properties and structural integrity of epoxy graphite composite materials are generally resistant to moisture, but physical and chemical degradation by ultraviolet light may be aggravated by water exposure. Ultraviolet degradation of the surface of a composite material can provide pathways for ingress of moisture and chemical agents. Moreover, the presence of moisture can enhance photo-oxidation reactions resulting in polymer scission or crosslinking. Water vapor or condensation can also remove soluble products of photo-oxidation reactions from a UV-irradiated surface and thereby expose fresh surfaces susceptible to further degradation.
Components made from epoxy graphite composite materials that are used in manufacturing are therefore often protected from exposure to ultraviolet light, e.g., with a coating of primer. The production of composites is usually very expensive, requiring expert and skilled personnel. Despite prudent manufacturing techniques, it often happens that components made from epoxy graphite composite materials begin to degrade before being used or during a manufacturing process if conditions are not controlled, as evidenced by the appearance of the characteristic green surface color. Once the surface of a composite material begins to degrade, its surface properties become altered. For example, it is difficult to apply paint to a UV-degraded surface because of the loss of surface properties.
For small components, limited amounts of UV-damaged surface may be restored with sand paper or washing with a solvent-soaked rag, but such methods are not appropriate for large-scale industrial use. For example, it would be impractical to apply liquids to the underside of an airplane wing component (having a very large surface area) because dripping liquid would contaminate the work area, and the usefulness of such methods are limited by rapid solvent evaporation. Large volumes of free-flowing (i.e., liquid) organic solvents are undesirable because of the risk of fire or environmental contamination, protection of personnel, and the need for hazardous waste disposal. In order to be practical, any method of removing UV-degradation products should ideally conclude with a water-based wash step, such as washing with soap and water. Furthermore, the method should be applicable and practical for horizontal, vertical, and overhead surfaces.
Accordingly, a need exists for a method for restoring large surfaces of an epoxy graphite composite material that has been degraded by ultraviolet radiation.